A configuration of a conventional receiver will be described with reference to FIG. 6.
An orthogonal frequency-division multiplexing (OFDM) modulation signal received by an antenna 601 is inputted to an automatic gain control (AGC) unit 602 and gain-adjusted based on a gain control signal from a loop filter 606 such that an output signal level becomes a predetermined value. The level-adjusted received signal which is an output of the AGC unit 602 is outputted to a frequency converter 603 and a detector 604.
The detector 604 detects the level of the input signal and outputs the signal to a target comparator 605. This signal is hereinafter referred to as a detection signal.
The target comparator 605 compares the input value with a preset target value, generates a gain control signal based on the comparison result to make the detection signal close to the target value, and outputs the gain control signal to the loop filter 606. The loop filter 606 extracts a low frequency component of the input signal and outputs the low frequency component to the AGC unit 602. The loop filter is used for the purpose of suppressing oscillation.
The frequency converter 603 converts the frequency of the input signal into an intermediate frequency (IF) band, performs channel selection filtering to pass the frequency band of one channel, and outputs the filtered signal to an analog-to-digital converter (ADC) unit 607. The ADC unit 607 converts the signal inputted from the frequency converter 603 into a digital signal and outputs the digital signal to a fast Fourier transform (FFT) unit 608. In this case, a sampling rate is determined by OFDM parameters (the number of FFT points and an effective symbol length).
The FFT unit 608 converts the input signal into a signal of a frequency domain and outputs the converted signal to an equalizer 609. The equalizer 609 performs equalization processing for correcting the amplitude and phase of the input signal, and outputs the equalization result to a determination unit 610. The determination unit 610 determines the input signal, associates the determination result with an error correction likelihood, and outputs the resultant signal to a forward error correction (FEC) unit 611. The FEC unit 611 performs de-interleaving processing on the input signal, and performs error correction of the signal based on the error correction likelihood for the determination result.
As another related art, for example, in Patent Document 1, each system for received signal processing includes an amplifier, an AGC circuit for automatic gain control having a different response speed, and an ADC. A digital signal processor which receives the signals having passed through the systems selects digital data having the lowest error rate from among the systems. Thus, it is possible to obtain the received signal data having the lowest error rate. For example, it discloses a technique for selecting an output of the system having the AGC circuit with a fast response speed when the received signal undergoes interference of fast level variations such as fading, and selecting an output of the system having the AGC circuit with a slow response speed when the received signal uses a modulation scheme with an amplitude component.
Patent Document 1: Japanese Patent Application Publication No. 2004-201066
Patent Document 2: Japanese Patent Application Publication No. 2010-130630
Patent Document 3: Japanese Patent Application Publication No. 2010-45706
Patent Document 4: Japanese Patent Application Publication No. 2006-319608
The problems in the above-mentioned related art will be described with reference to FIGS. 7A to 7E. FIGS. 7A to 7E are diagrams for explaining the adverse effects of interferences in the receiver.
In FIG. 7A shows the power spectrum when large-level interference due to, e.g., a radar appears in the same band as the desired wave. Further, FIG. 7B represents the reception level when the interference is mixed repeatedly and intermittently. In the conventional receiver of FIG. 6, it is difficult for the gain control signal to follow the level variation at a high speed.
In FIG. 7C shows a graph of an ideal gain control signal and a conventional gain control signal. The ideal gain control signal controls the gain so as not to cause intermodulation distortion of an analog element by lowering the gain control signal immediately even when interference is mixed, and returns the gain control signal to an original state immediately even when interference is no longer mixed to prevent desensitization in the analog element.
However in a feasible receiver, if a time constant of the loop filter is shortened by allowing the gain control signal to follow at a high speed, oscillation may occur. Accordingly, the time constant cannot be shortened. Thus, as shown by a dotted line in FIG. 7C, both when the interference is mixed and when the interference is no longer mixed, the gain control signal deviates from the ideal value. Thus, since the gain control cannot follow at a high speed, the level of the received signal when the interference is mixed reaches a nonlinear region of the analog element to cause intermodulation distortion in the frequency converter 603, thereby resulting in deterioration of the transmission quality.